Sodium-ion batteries (SIBs) have garnered significant attention as a promising alternative to lithium-ion batteries (LIBs) due to the abundance and cost-effectiveness of sodium resources. However, the development of high-performance solid-state electrolytes remains a major challenge in the advancement of SIBs. NASICON-type solid electrolytes, specifically Na₃Zr₂Si₂PO₁2 (NZSP), are known for their excellent ionic conductivity and structural stability, making them ideal candidates for use in SIBs. To further enhance their ionic conductivity, one common strategy is to dope low-valence cations into the structure, potentially replacing Zr⁴⁺ ions and increasing the concentration of Na⁺ ions. Despite the widespread use of Mg²⁺ doping in NASICON materials to enhance ionic conductivity, the underlying mechanism remains underexplored. Therefore, the aim of this study is to investigate the effect of Mg²⁺ doping on the microstructure and ionic conductivity of Na_3+2xMg_xZr_2-xSi₂PO₁₂ (NMZSPx) solid-state electrolyte ceramics and elucidate the underlying mechanism of ionic conductivity enhancement.

In this study, a series of Mg²⁺ doping Na_3+2xMg_xZr_2-xSi₂PO₁₂ (NMZSPx, 0 ≤ x ≤ 0.20) ceramics were synthesized via a traditional sintering method. The samples were characterized using several techniques, providing insights into the electrical properties and conductivity mechanisms. Electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the ionic conductivities of the samples across a range of temperatures. X-ray diffraction (XRD) analysis was employed to investigate the phase composition and crystalline structure of the samples. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy, including ³¹P, ²⁹Si, and ²³Na Magic Angle Spinning (MAS) NMR, was used to analyze the local atomic environments and determine the distribution of different elements in the ceramics. Scanning electron microscopy (SEM) was used to examine the microstructure of the ceramics, with individual elemental maps to assess the distribution of Na, Mg, Zr, Si, P, and O elements.

The results of this study reveal that Mg doping significantly affects both the microstructure and the ionic conductivity of Na_3+2xMg_xZr_2-xSi₂PO₁₂ ceramics. As the Mg²⁺ doping concentration increases, the ionic conductivity initially increases, reaching a peak value at x = 0.12, with a conductivity of 1.54 × 10^-3 S·cm^-1 at 25 ℃, which is 3.13 times higher than that of the undoped sample. However, further doping beyond x = 0.12 leads to a decrease in conductivity, indicating an optimal doping concentration for ionic conductivity enhancement. XRD analysis indicates that the samples with doping amount x > 0.12 exhibit a characteristic diffraction peak of rhombohedral crystal at 30.2°. This indicates that as the amount of Mg²⁺ doping increases, the NASICON main phase of NMZSPx samples gradually transitions from monoclinic phase to rhombohedral phase. Mg²⁺ ions do not replace Zr⁴⁺ ions in the NASICON main phase as previously assumed. Instead, a large portion of Mg²⁺ ions preferentially forms sodium magnesium phosphate heterogeneous phases at the grain boundaries. This observation is consistent with the results from SSNMR, where the chemical shifts of those signal peaks in the ³¹P, ²⁹Si, and ²³Na MAS NMR spectra show significant changes in the local environments of these elements, reflecting the influence of Mg²⁺ doping on the phase composition and atomic distribution. The formation of these magnesium phosphate secondary phases indirectly increases the concentration of Na⁺ by altering the Si/P ratio in the NASICON structure. However, the increase in Na⁺ concentration is not the primary reason for the enhanced ionic conductivity. The key factor contributing to the increased ionic conductivity is the improvement in the microstructure of the NMZSPx ceramics. SEM images show that appropriate Mg²⁺ doping significantly reduces the porosity of the ceramic samples and enhances the connectivity at the grain boundaries, facilitating easier Na⁺ transport. Specifically, x = 0.12 sample exhibits the densest microstructure, with good contact between grains and no noticeable defects, resulting in the highest ionic conductivity. However, excessive Mg²+doping (x > 0.12) leads to the formation of more impurity phases and increased porosity, which hinders ion transport across the grain boundaries, resulting in a decrease in ionic conductivity. This suggests that while Mg²⁺ doping can improve the ceramics' microstructure and reduce grain boundary resistance up to a certain point, excessive doping leads to the formation of impurity phases that impede ionic conduction.

This study demonstrates that Mg²⁺ doping in Na_3+2xMg_xZr_2-xSi₂PO₁₂ ceramics enhances ionic conductivity by improving the microstructure, particularly by reducing porosity and enhancing grain boundary connectivity. The optimal doping concentration of Mg²⁺ ions is found to be x = 0.12, which exhibits the highest ionic conductivity. However, excessive Mg²⁺ doping leads to the formation of sodium magnesium phosphate heterogeneous phases and increased porosity, which hinders ion transport and reduces the overall conductivity. The results also suggest that Mg²⁺ ions primarily influence the NMZSPx ceramics' conductivity by modifying the microstructure and inducing the formation of impurity phases at the grain boundaries, rather than directly substituting for Zr⁴⁺ ions in the NASICON structure. These findings provide new insights into the role of Mg²⁺ doping in enhancing the ionic conductivity of NASICON-type solid-state electrolytes and highlight the importance of optimizing doping concentrations to balance the formation of impurity phases and the improvement of microstructural properties.